It is common to use a single antenna array to provide a radiation pattern, or beam, which is steerable. For example, steerable beams are often produced by a linear planar array of antenna elements each excited by a signal having a predetermined phase differential so as to produce a composite radiation pattern having a predefined shape and direction. In order to steer this composite beam, the phase differential between the antenna elements is adjusted to affect the composite radiation pattern. A multiple beam antenna array may be created through the use of predetermined sets of phase differentials, where each set of phase differential defines a beam of the multiple beam antenna.
There are a number of methods of beam steering using matrix type beam forming networks, such as a Butler matrix, that can be made to adjust parameters, such as, for example, might be directed from a computer algorithm. This is the basis for adaptive arrays.
When a linear planar array is excited uniformly (uniform aperture distribution) to produce a broadsided beam projection, the composite aperture distribution resembles a rectangular shape. When this shape is Fourier transformed in space, the resultant pattern is laden with high level side lobes relative to the main lobe. Moreover, as the beam steering increases, i.e., the beam is directed further away from the broadside, these side lobes grow to higher levels.
These side lobes act to degrade the performance of the antenna system by making it responsive to signals in an undesired direction, potentially interfering with the desired signal. Therefore, in most practical applications these high level side lobes are an undesirable side effect.
Additionally, broadside excitation of a planar array yields maximum aperture projection. Accordingly, when such an antenna is made to come off the normal axis, i.e., steered away from the broadside position which is normal to the ground surface and centered to the surface itself, the projected aperture area decreases causing a scan loss. This scan loss further aggravates the problems associated with the increased side lobes because not only is the aperture area of the steered beam decreased due to the effects of scan loss, but the unwanted side lobes are simultaneously increased due to the effects of beam steering.
One prior art attempt to control these undesired side lobes has been to restrict the horizontal spacing between the various antenna elements making up the planar array to a spacing of less than 1/2.lambda. between the elements. However, such antenna element placement has had limited success.
Another prior art attempt to control these side lobes has been to utilize non-uniform aperture distribution, such as raised cosine aperture distribution. However, this technique results in beam broadening and lower maximum gain.
Accordingly, a need exists in the art for an antenna system which provides for uniform aperture distribution without producing undesirable high level side lobes. Moreover, a need exists in the art for such a system to produce acceptable side lobe levels when the beam comes away from the broadside.
A further need exists in the art for an antenna system which does not rely on inter-element spacing of less than 1/2.lambda. to reduce undesired side lobes.
These and other objects and desires are achieved by an antenna design which utilizes parasitic elements placed at predetermined locations among the active elements to provide an improved radiation pattern with reduced side lobes.